The lipooligosaccharides (LOSs) of Neisseria gonorrhoeae are important pathogenic and antigenic outer membrane components. LOSs are cytotoxic and damage human fallopian tubes. Antibodies against LOS develop in response to disseminated gonococcal infection, and therefore, LOS are important in the host immune response against gonococci. Each gonococcal strain may produce several structurally different LOS components. In addition, a recent study in human volunteers indicates that gonococcal LOS expression can shift with the stage of infection. Thus not only can LOS structure vary between strains, but each strain can produce a variety of LOSs, depending on its environment. Changes in LOS structure can affect the interaction of LOS with host defenses. In particular, gonococci utilize exogenous cytidine monophospho-N-acetylneuraminic acid (CMP-NANA) and sialylate a specific LOS component in vitro. Sialylated gonococci become resistant to complement mediated killing. We have determined the structures of some prevalent LOS components produced by clinical gonococcal isolates and revealed that gonococci produce higher MW LOSs by elongating the carbohydrate sequence of a small MW LOS in vivo. However, gonococci produce several other unknown LOS components. To understand the complex LOS biosynthesis in vivo and in vitro, further structural determinations of gonococcal LOSs are essential. For this purpose, we have selected two strains PID-2 and 15253. Strain PID-2, isolated from a patient with pelvic inflammatory disease, produces six LOS components similar to those produced by urethral gonorrhea. The structures of several PID-2 LOS components are unknown. Strain 15253, isolated from a patient with disseminated infection, produces a single LOS component recognized by MAb 3G9. We will determine the structures of these LOS components to further understand the complex antigenicity of gonococcal LOS. In addition, we will determine the structures of the sialylated gonococcal LOSs. We will also develop monoclonal antibodies (MAbs)specific for sialylated gonococcal LOSs to characterize the types of sialylation that may take place in vivo. Information obtained with this study will provide the structural basis for the in vivo sialylation of LOS and the resulting change in serum sensitivity. Our understanding of the biology of gonococcal LOS is incomplete. Understanding LOS expression in the host environment and characterization of LOS epitopes on gonococci is critical to understand the pathogenicity and immunogencity of gonococci.